An Influenza pandemic remains an acute threat to world health; and stockpiling a universally protective Influenza vaccine provides a strong defense against this potential catastrophe. The hemagglutinin (HA) protein is the primary target of humoral Ab responses to Influenza; the majority of broadly protective mAbs (bnAbs) against influenza isolated from humans recognize conserved and conformation-specific epitopes in the HA Stalk. But, strain-specific, immunodominant epitopes in the Head of HA overwhelm immune responses to the Stalk. An HA immunogen from which the Head domain has been removed will elicit anti-Stalk antibodies that protect broadly against seasonal, as well as pandemic, Influenza. Despite significant progress, it has so far not proved possible to design a stable Headless HA that assumes its fully native conformation and thereby elicits universally protective Ab responses. None of the most promising, recent designs have progressed to clinical development. Avatar has developed a strategy to produce a conformationally intact Headless HA that overcomes these limitations. This strategy involves first locking the structure of the conserved Stalk with target dityrosine (DT) crosslinks, so that it can no longer lose its native conformation. Then we remove the variable and immunodominant Head domain with a site-specific protease, using engineered recognition sites. This DT- Headless HA immunogen is in its fully native conformation, and responses to this more perfect immunogen will improve Ab titers and affinities to conserved epitopes, and thus protect broadly against all strains of Influenza. In Phase I, we successfully designed and characterized our DT-Headless HA (AI118087). In Phase II we will demonstrate heterologous protection in Balb/c mouse challenge studies, and confirm efficacy of this product in ferret challenge studies (Aims 1 & 2). We will also transfer our design into a Group II HA and test its heterologous protection in mouse and ferret lethal challenge studies (Aim 3 & 4). By inducing higher avidity, higher titer Ab responses to the conserved Stalk, Avatar's Headless HA immunogen will give rise to broad protection against homologous and drift variants, as well as Group 1 & 2 heterologous challenge. We will compare the results obtained with our Group I and II Headless HA immunogens and select a product candidate for preclinical and clinical development.